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VON  HTOIBOLDT.                183
experiences led him.  to  the  conclusion  that  earth-
quakes were   always  taking place  every minute in
some portion of the earth ; that therefore the interior
of the earth was constantly reacting upon the exte-
rior, that probably the high degree of heat of the
earthy masses in their depth,  where they exist in a
molten condition, is  the  cause; and that therefore
the agitation  is  not  confined   to   certain species of
mountains, but may occur on any spot of the earth.
But from, the observance of simultaneous distant agi-
tations, from the direction of these earthquakes and
waves which frequently cross each  other, from the
subterraneous noise which is often heard at miles dis-
tant from the active volcano or the revolutionary por-
tion of the earth's surface, Humboldt recognised cer-
tain subterranean connected veins of volcanic activity,,
having their safety valves in the eruptive volcanos,
and either discharging their explosive masses from
the crater,  or, if these craters are choked, finding
another, and, for the inhabitants, a more dangerous:
outbreak for their fluids expanded by heat.    Some-
times they are not ripe for eruption, and only pro-
duce earthquakes.    Electric fluids, expanded by heaty
are, according to Humboldt, the cause of all volcanic
phenomena, from the  faintest vibration to the most
terrible eruption.    These expanded vapours of boiling
water,  or melted metals and rocks, rising, roll along
the volcanic galleries in the earth's interior, often find
their outlets choked with crystallized or cooled masses*
.and then the pressure of expansion raises the earth
or imparts the waving motion of the elastic fluids to
the more solid mass.
But Humboldt also recognised the chemical changes
in the earth's surface and in the atmosphere, as caused
by the inner vital heat of our earth. The vapoiars
exhaled the oxygen gas which the earth discharges
into the atmosphere itself almost free from nitrogen;
many other peculiar gases which rise from various
clefts in the earth seemed to Humboldt the evident
proofs of a constant burning process existing in the